In recent years, from the viewpoint of environmental protection, various electric apparatuses have been controlled by inverter circuits to save energy and achieve higher efficiency. In particular, the automobile industry has been actively developing environmental-friendly, energy-saving, and highly-efficient techniques. These techniques put hybrid vehicles (hereinafter, referred to as HEVs) that are driven with an electric motor and an engine, in the market.
The electric motor for the HEVs is operated in a high voltage range of several hundred volts. Accordingly, a film capacitor having electric characteristics, such as high withstanding voltage with low loss, has attracted attention as a capacitor to be used for the electric motor.
In general, film capacitors are roughly separated into two groups: capacitors including electrodes made of metal foil; and capacitors including electrodes made of metal films, e.g. vapor-deposited on dielectric films. The capacitors including the vapor-deposited metal film electrodes has a smaller volume occupied by the electrodes than the capacitors including the metal foil electrodes, consequently having a small size and a light weight. The vapor-deposited electrode layers have a self-recovery function, generally referred to as a self-healing property, in which a portion around a defective portion including insulation failure vaporizes and scatters, so that the capacitor recovers its function. This provides high reliability against dielectric breakdown. A thinner electrode layer is more likely to vaporize and scatter, which improves the self-healing property. As a result, higher withstanding voltage can be obtained.
FIG. 7 is a perspective view of capacitor element 1 of a conventional film capacitor. Capacitor element 1 includes a pair of metalized films 4 each having dielectric film 2 and electrode layer 3 disposed on dielectric film 2. Metalized films 4 are stacked and wound such that respective electrode layers 3 face each other across dielectric film 2. A pair of external electrodes 6 is disposed on different end faces of capacitor element 1.
Each of electrode layers 3 is divided into small electrode segments 7 which are interconnected with fuses 8. If a segment out of small electrode segments 7 is short-circuited and overcurrent is caused, fuses 8 connected to short-circuited small electrode segment 7 melts and separates this small electrode segment 7 from the other of small electrode segments 7. This recovers insulation of overall small electrode segments 7. Although such an operation slightly reduces the capacitance, high withstanding voltage is maintained.
A resin film is used as each of dielectric films 2. Examples of the resin film include polypropylene, polyethylene-terephthalate, polyethylene-naphthalate, and polyphenyle-sulfide. In recent years, in order to increase heat resistance property, a styrene polymer having a syndiotactic structure has started to be used as a new film material.
PTL 1 discloses a conventional film capacitor which includes a film containing a styrene polymer having a syndiotactic structure.